Greeting non-Muslims at the time of their religious celebrations saying: “Have a jolly day” or “Congratulations on this day” is impermissible. If extending these greetings is not kufr, then at least, it is prohibited. This is worse than congratulating a non-Muslim for consuming alcohol, committing murder, or having unlawful sex; rather, it is like congratulating him for prostrating to the cross, yet many Muslims who have weak faith do so without understanding how repugnant this action really is.